Editing Radiocarbon dating (section)

===Reservoir effects===
Libby's original exchange reservoir hypothesis assumed that the {{chem|14|C}}/{{chem|12|C}} ratio in the exchange reservoir is constant all over the world,<ref name=Libby1965>Libby (1965), p. 6.</ref> but it has since been discovered that there are several causes of variation in the ratio across the reservoir.<ref name=Bowman1995/>

====Marine effect====
The {{chem|CO|2}} in the atmosphere transfers to the ocean by dissolving in the surface water as carbonate and bicarbonate ions; at the same time the carbonate ions in the water are returning to the air as {{chem|CO|2}}.<ref name=Libby1965/> This exchange process brings {{chem|14|C}} from the atmosphere into the surface waters of the ocean, but the {{chem|14|C}} thus introduced takes a long time to percolate through the entire volume of the ocean. The deepest parts of the ocean mix very slowly with the surface waters, and the mixing is uneven. The main mechanism that brings deep water to the surface is upwelling, which is more common in regions closer to the equator. Upwelling is also influenced by factors such as the topography of the local ocean bottom and coastlines, the climate, and wind patterns. Overall, the mixing of deep and surface waters takes far longer than the mixing of atmospheric {{chem|CO|2}} with the surface waters, and as a result water from some deep ocean areas has an apparent radiocarbon age of several thousand years. Upwelling mixes this "old" water with the surface water, giving the surface water an apparent age of about several hundred years (after correcting for fractionation).<ref name=Bowman1995/> This effect is not uniform&nbsp;– the average effect is about 400 years, but there are local deviations of several hundred years for areas that are geographically close to each other.<ref name=Bowman1995/><ref name=Cronin2010/> These deviations can be accounted for in calibration, and users of software such as CALIB can provide as an input the appropriate correction for the location of their samples.<ref name=Alves2018>{{cite journal|last1=Queiroz-Alves|first1=Eduardo|last2=Macario|first2=Kita |last3=Ascough|first3=Philippa |last4=Bronk Ramsey|first4=Christopher |year=2018|title=The worldwide marine radiocarbon reservoir effect: Definitions, mechanisms and prospects|journal=Reviews of Geophysics |volume=56|issue=1|pages=278–305|doi=10.1002/2017RG000588|bibcode=2018RvGeo..56..278A| s2cid=59153548 |url=http://eprints.gla.ac.uk/160036/7/160036.pdf}}</ref> The effect also applies to marine organisms such as shells, and marine mammals such as whales and seals, which have radiocarbon ages that appear to be hundreds of years old.<ref name=Bowman1995/>

====Hemisphere effect====
The northern and southern hemispheres have [[atmospheric circulation]] systems that are sufficiently independent of each other that there is a noticeable time lag in mixing between the two. The atmospheric {{chem|14|C}}/{{chem|12|C}} ratio is lower in the southern hemisphere, with an apparent additional age of about 40 years for radiocarbon results from the south as compared to the north.{{#tag:ref|Two recent estimates included 8–80 radiocarbon years over the last 1000 years, with an average of 41 ± 14 years; and −2 to 83 radiocarbon years over the last 2000 years, with an average of 44 ± 17 years. For older datasets an offset of about 50 years has been estimated.<ref name=Hoggetal/>|group=note}} This is because the greater surface area of ocean in the southern hemisphere means that there is more carbon exchanged between the ocean and the atmosphere than in the north. Since the surface ocean is depleted in {{chem|14|C}} because of the marine effect, {{chem|14|C}} is removed from the southern atmosphere more quickly than in the north.<ref name=Bowman1995/><ref name=Hoggetal>{{Cite journal | last1=Hogg | first1=A.G. | last2=Hua | first2=Q. | last3=Blackwell | first3=P.G. | last4=Niu | first4=M. | last5=Buck | first5=C.E. | last6=Guilderson | first6=T.P. | last7=Heaton | first7=T.J. | last8=Palmer | first8=J.G. | last9=Reimer | first9=P.J. | last10=Reimer | first10=R.W. | last11=Turney | first11=C.S.M. | last12=Zimmerman | first12=S.R.H. | date=2013 | title=SHCal13 Southern Hemisphere Calibration, 0–50,000 Years cal BP | journal=Radiocarbon | volume=55 | issue=4 | pages=1889–1903 | doi=10.2458/azu_js_rc.55.16783| s2cid=59269731 | doi-access=free | bibcode=2013Radcb..55.1889H | hdl=10289/7799 | hdl-access=free }}</ref> The effect is strengthened by strong upwelling around Antarctica.<ref name="Russel" />

====Other effects====
If the carbon in freshwater is partly acquired from aged carbon, such as rocks, then the result will be a reduction in the {{chem|14|C}}/{{chem|12|C}} ratio in the water. For example, rivers that pass over [[limestone]], which is mostly composed of [[calcium carbonate]], will acquire carbonate ions. Similarly, groundwater can contain carbon derived from the rocks through which it has passed. These rocks are usually so old that they no longer contain any measurable {{chem|14|C}}, so this carbon lowers the {{chem|14|C}}/{{chem|12|C}} ratio of the water it enters, which can lead to apparent ages of thousands of years for both the affected water and the plants and freshwater organisms that live in it.<ref name=Aitken1990/> This is known as the [[hard water]] effect because it is often associated with calcium ions, which are characteristic of hard water; other sources of carbon such as [[humus]] can produce similar results, and can also reduce the apparent age if they are of more recent origin than the sample.<ref name=Bowman1995/> The effect varies greatly and there is no general offset that can be applied; additional research is usually needed to determine the size of the offset, for example by comparing the radiocarbon age of deposited freshwater shells with associated organic material.<ref>Taylor & Bar-Yosef (2014), pp. 74–75.</ref>

[[Volcanic eruptions]] eject large amounts of carbon into the air. The carbon is of geological origin and has no detectable {{chem|14|C}}, so the {{chem|14|C}}/{{chem|12|C}} ratio near the volcano is depressed relative to surrounding areas. Dormant volcanoes can also emit aged carbon. Plants that photosynthesize this carbon also have lower {{chem|14|C}}/{{chem|12|C}} ratios: for example, plants in the neighbourhood of the [[Furnas]] caldera in the [[Azores]] were found to have apparent ages that ranged from 250 years to 3320 years.<ref>{{Cite journal |last1=Pasquier-Cardin |first1=Aline |last2=Allard |first2=Patrick |last3=Ferreira |first3=Teresa |last4=Hatte |first4=Christine |last5=Coutinho |first5=Rui |last6=Fontugne |first6=Michel |last7=Jaudon |first7=Michel |date=1999 |title=Magma-derived {{chem|14|C|O|2}} emissions recorded in {{chem|14|C}} and {{chem|13|C}} content of plants growing in Furnas caldera, Azores |journal=Journal of Volcanology and Geothermal Research |volume=92 |issue=1–2 |pages=200–201|doi=10.1016/S0377-0273(99)00076-1 }}</ref>